v3.26.1
Accumulated Other Comprehensive Income (Loss) - Summary of Changes in Accumulated Other Comprehensive Income (Detail) - USD ($)
$ in Millions
 3 Months Ended
Mar. 31, 2026
Mar. 31, 2025
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]    
Beginning balance $ 7,945.9 $ 6,707.5
Net current-period other comprehensive income (111.3) 55.2
Ending balance 7,908.1 7,214.5
Unrealized holding gains (losses) on AFS securities    
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]    
Beginning balance (340.7) (534.7)
Other comprehensive (loss) income before reclassifications (74.2) 55.7
Amounts reclassified from AOCI (37.8) (1.6)
Net current-period other comprehensive income (112.0) 54.1
Ending balance (452.7) (480.6)
Unrealized holding gains (losses) on junior subordinated debt    
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]    
Beginning balance (3.5) 1.4
Other comprehensive (loss) income before reclassifications 1.0 1.1
Amounts reclassified from AOCI 0.0 0.0
Net current-period other comprehensive income 1.0 1.1
Ending balance (2.5) 2.5
Accumulated Other Comprehensive Income (Loss)    
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]    
Beginning balance (344.2) (533.7)
Other comprehensive (loss) income before reclassifications (73.5) 56.8
Amounts reclassified from AOCI (37.8) (1.6)
Net current-period other comprehensive income (111.3) 55.2
Ending balance (455.5) (478.5)
Unrealized holding gains (losses) on SERP | Unrealized holding gains (losses) on SERP    
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]    
Beginning balance 0.0 (0.4)
Other comprehensive (loss) income before reclassifications (0.3) 0.0
Amounts reclassified from AOCI 0.0 0.0
Net current-period other comprehensive income (0.3) 0.0
Ending balance $ (0.3) $ (0.4)